Glaucoma is one of the diseases commonly screened for when a patient visits an optometrist or opthalmologist. In the United States there are approximately 4 million people affected by this disease.
Several diagnostic tests are commonly used for diagnosing glaucoma. These include measurement of intraocular pressure (IOP), visual field checks using a perimeter, and, less frequently, stereo fundus photography for subjective evaluation of the optic nerve head.
The most common screening test is the IOP test, since glaucoma is usually accompanied by an increased intraocular pressure. However, this test, cannot be reliably used to diagnose glaucoma because increased IOP does not necessarily indicate that glaucoma is present. Further, a patient may have so-called "low-IOP glaucoma" in which glaucoma is present without a significantly increased IOP. Thus, the IOP test does not identify all cases of glaucoma. It is used because there is nothing better available at a cost low enough for the majority of opthalmic offices to absorb.
The visual field check using a perimeter can be used to reliably diagnose glaucoma, since loss of visual field eventually occurs as part of the onset of the disease. However, by the time glaucoma can be detected with a perimeter, the disease is well advanced and, although the patient's eyesight can be stabilized at the current level using medication, the visual field already lost is usually not recoverable. Thus, the perimeter check alone, while accurate and inexpensive, does not provide a sufficiently early diagnosis to prevent significant loss of eyesight.
It is well documented in the literature that structural changes in the optic nerve head, which is located at the back of the eye (at the ocular fundus), may be seen by an opthalmologist before the IOP and visual field tests indicate that a patient has glaucoma. As glaucoma progresses, the volume of the optic nerve head increases. In addition, a cupping or excavation may appear in the optic nerve head as the nerve tissue is damaged by the onset of the disease. If these volumetric changes could be detected early enough in a reliable and reproducable manner, then glaucoma could be detected earlier in patients and the loss of eyesight could be prevented. Typically, however, because the IOP and visual field checks are the only widely available glaucoma diagnostic tools, a patient is not diagnosed as having glaucoma until irreversible visual field loss has already occurred. As noted, structural changes to the optic nerve head may presently be clinically identified on a qualitative level, but there is a need for a reliable method for accurately and quantitatively measuring changes in the topography of the optic nerve head.
Of course, other problems may also be detected by measuring volumetric changes in the optic nerve head. For example, brain tumors or fundus tumors can cause the optic disc to decrease in volume.
Several prior art systems have attempted to analyze changes in the optic nerve head for disease diagnosis U.S. Pat. No. 4,423,931 to Shapiro shows a stripe projection accessory for a fundus camera. The apparatus projects a fixed stripe pattern onto the fundus, which is photographed. The specification of this patent indicates that the photographic image produced could be digitized and then analyzed by a computer to identify changes in the structure of the fundus. However, the system disclosed is expensive to produce because it requires projecting a stripe pattern with a large depth of field. Apparently, the modes of analysis referred to by Shapiro could not operate accurately unless the stripe pattern was precisely focussed over the entire optic nerve head. Also, devices of this type suffer repeatability problems since they must be reattached to the fundus camera for each use, producing possible alignment errors that would prevent precise comparison of photos taken at different sittings.
U.S. Pat. No. 4,715,703 to Cornsweet et al. discloses an apparatus for examining an ocular fundus which projects light of variable wavelengths onto the fundus and then uses a stereo camera arrangement to collect data about the fundus. The data is stored digitally and can be overlaid on-screen to show changes in the fundus. An algorithm cross-correlates the stereo images to determine depth information. However, because of the complexity of the optics and multiple camera systems used, such systems are fairly expensive to produce. Further, such systems in practice have not provided the high degree of repeatability and accuracy that is required for clinical use. One significant problem with systems of this type is that any variation in the angle of separation between the stereo image pair will produce a false apparent variation in the depth of the surface being imaged.
In general, the prior art systems that have projected lines or spots on the optic nerve head are limited in that they require sharp focussing of light on the optic nerve head tissue to obtain accurate topographic data. In practice, it is difficult to sharply focus light over the entire optic nerve head since the optic nerve head varies in depth and because the optic nerve head tissue is relatively clear and thus tends to absorb and diffuse the light projected on it. Expensive optical systems were employed in the prior art systems in an attempt to compensate for this characteristic of the retina. However, even complex optics did not provide a solution to the problem. Systems that rely on the focus of lines or laser spots to obtain depth information are inherently flawed in that the persons most at risk for glaucoma tend to have other problems associated with aging eyes. Light from lines or laser spots beamed through an opaque portion of the cornea or lens such as a cateract will be scattered, and accurate depth data will not be obtained if the operation of the system depends on the sharp focus of this light on the optic nerve head. As a result, prior art systems that require accurate focus will not provide accurate optic nerve head depth measurements for the people most at risk.
Additionally, prior-art methods of analysis which require manual examination and comparison of photographic images are time consuming and subjective and are therefore less practical in a clinical environment.
U.S. Pat. No. 4,732,466 to Humphrey discloses a fundus camera which uses a rotating drum which scans an illuminated region across the retina to form an image on a vidicon tube. This system does not perform analysis of the image produced and has moving parts which are less reliable than fixed optoelectronics.
Further systems form ocular images using point light sources such as scanning lasers. Systems of this type are disclosed in U.S. Pat. No. 4,900,144 to Kobayashi, U.S. Pat. No. 4,867,554 to Matsumura, U.S. Pat. No. 4,579,430 to Bille, and U.S. Pat. No. 4,728,196 to Gerstorfer. These systems, like the prior-art line projection systems described previously, are expensive to produce, difficult to adjust to produce repeatable output, and depend on accurate focussing of the laser spots.
U.S. Pat. No. 4,863,260 to Gersten shows a system for topographical modeling of anatomical surfaces, such as the cornea of the eye. The system projects illuminated mires onto the cornea and a video camera transmits the image to a computer. The image is then radially scanned by the computer to provide desired information. The system disclosed is not adapted to compare images or to diagnose diseases such as glaucoma, and is not adapted for determining the topography of internal ocular surfaces such as the optic nerve head.
All of these systems known to the present inventors fail to provide a system for detecting glaucoma and performing other ocular structure diagnosis which is accurate, affordable, and can be used as a regular part of a daily clinical opthalmic practice without an adverse effect on patient throughput. What is needed is a system which has relatively simple and inexpensive optic components and which can collect, in a highly automated and repeatable fashion, accurate topographical information about the retina, and then compare the information collected to stored, previously collected topographic information for the same patient to quantitatively identify changes in the retinal topography. Most importantly, such a system should not rely on accurate focussing of light beams on the retina since characteristics of aging corneas make precise focusing virtually impossible in the patients most at risk for glaucoma.